High strength, air-hardening steel with excellent shaping properties

ABSTRACT

The invention relates to a high strength, air-hardening, temper-resistant steel, which can easily be welded and galvanized and exhibits excellent shaping properties, particularly for the construction of lightweight vehicles. The inventive steel comprises the following elements (contents in mass %): C0.07 to ≦0.15, Al≦0.05, Si 0.15 to ≦0.30, Mn 1.60 to ≦2.10, P≦0.020, S≦0.010, N 0.0030 to ≦0.0150, Cr 0.50 to ≦1.0, Mo 0.30 to ≦0.60, Ti 0.010 to ≦0.050, V 0.12 to ≦0.20, B 0.0015 to ≦0.0040, remainder iron including incidental steel-accompanying elements.

The invention relates to a high strength, air-hardening steel withexcellent shaping properties, in particular for the construction oflightweight vehicles according to the preamble of claim 1.

The hotly contested automobile market forces the manufacturer, i.a., toconstantly seek solutions for lowering the fleet consumption while stillmaintaining a highest possible comfort and greatest possible protectionof occupants. Weight saving of all vehicle components plays hereby arole, on one hand, as is also, on the other hand, a substantiallyeffective behavior of the individual structures, when exposed to staticand dynamic stress during operation as well as in the event of a crash.Suppliers attempt to account for this requirement by reducing the wallthickness of available high strength and super high strength steel whileat the same time improving the behavior of the structures duringmanufacture and operation. Such steels are thus required to satisfy thecomparably high demands with respect to strength, ductility, toughness,energy absorption, and workability, for example through cold forming,welding, and/or surface treatment.

As a consequence of the high demands for corrosion protection, thesurfaces of these steels must be additionally treated with respectiveanti-corrosion coats, like, e.g., of zinc, whereby conventional hot dipgalvanizing as well as high-temperature galvanizing are used.

Apart from the afore-described general requirements, the followingmechanical characteristic values should be reached in the tempered stateby way of example:

R_(el) and R_(p0.2): 700-850[MPa] R_(m): 800-1000[MPa] A₈₀: ≧11 [%], andA₅: ≧13 [%]

In the past, mostly conventional steels of relative great metal sheetthickness, water-tempered high strength fine grained steels, polyphasesteels, or alternative materials, like aluminum, have been used for thisrange of application.

The use of conventional steels is hereby accompanied by the drawback ofthe substantial structure weight. Using super high strength polyphasesteels as an alternative has drawbacks, such as poorer welding tendencyand malleability as a consequence of the high base hardness.Water-tempered steels are expensive to manufacture and thus oftentimesuneconomically.

For these reasons, air-hardening steel materials have been developed asan alternative which overcome the shortcomings of conventional steelsjust by air-cooling the steel, following a heat treatment for example,in order to realize the demands on material properties.

When, after cold rolling, the steel is cooled by air, at least insections, at such a speed as to trigger the air hardening effect, thecold workability can be realized by a subsequent soft annealing process,e.g. in a hood-type annealing furnace, or by homogenizing throughannealing. As an alternative, cold workability may also be retainedafter hot rolling, when slowly cooling a coil that has been woundrespectively tight in some circumstances in a special heat-insulatedhood.

After cold forming or shaping, air hardening with subsequent temperingmay then be realized again by a following heat treatment, for exampleadvantageously by high temperature galvanizing.

The term “cold forming” relates hereby to the following processvariants:

-   a) the direct production of respective structures of hot strip    through deep drawing or the like with subsequent optional tempering    treatment.-   b) The further processing to tubes with respective drawing and    annealing processes. The tubes in turn are subsequently shaped and    then tempered to parts, e.g. through bending, internal high-pressure    forming (IHF) or the like.-   c) The further processing of the hot strip to cold strip with    integrated (hood) annealing process. The cold strip is then    subjected to deep drawing or the like, like in a).

Typical characteristic values for hot-annealed, cold-formableair-hardening steels for hot-formed or cold-formed metal sheets andtubes are listed hereinafter:

R_(el) and R_(p0.2): 330-500[MPa] R_(m): 480-620[MPa] A₈₀: ≧20 [%], andA₅: ≧22 [%]

DE 102 21487 B4 discloses the use of an air-hardening steel material forshaped parts in the construction of lightweight vehicles, containing theprimary elements C (0.09-0.13%), Si (0.15-0.30%), Mn (1.10-1.60%),Cr(1.0-1.6%), Mo (0.30-0.60%), and V (0.12-0.25%), remainder ironincluding incidental accompanying elements.

Although this alloy concept on the basis of Co—Mo—V is able to attainmechanical material properties as well as a good tempering resistanceand galvanizing capability, as demanded for the stated field ofapplication, there is the drawback of a relatively high Cr content of1.0-1.6%, that can cause unwanted chromium-carbide precipitations in theweld seam, in particular when tubes are produced by the predominantlyused high frequency induction (HFI) welding process. Theseprecipitations may lead to crack formation in the weld seam and thus toa premature failure of the structure, when the welded tube is furtherprocessed by shaping, or when the welded structure is exposed tosignificant mechanical stress during operation. The relatively highcontent of chromium also further increases costs.

EP 0 576 107 B1 discloses an air-hardening steel with reduced Cr contentfor production of seamless, ungalvanized structural tubes, for exampleas door reinforcements in the manufacture of automobiles. The alloyconcept on the basis of Mn—Si—Ti—B includes as primary elements C(0.15-0.30%), Mn (2.05-3.35%), Si (0.50-0.80%), Cr (0.5-1.0%), Mo (max.0.6%), Ti (0.01-0.05%), B (0.0015-0.0035%), and N (0.002-0.015%),remainder iron and incidental accompanying elements.

This steel, known for the seamless tube production, has the drawbackthat as a result of the relatively high contents of C and Mn, this alloyconcept places constraints on the basic welding capability of the steel,on one hand, and greatly limits the galvanizing capability through hotdipping or high temperature galvanizing as a result of the also highcontent of Si of up to 0.8%.

In-house test have further shown that the essential tempering resistanceof this conventional steel is not ensured in particular because of theabsence of vanadium so that the strength decreases significantly belowthe required values for air-hardened steel, when exposed to highertemperatures, e.g. ≧550° C., as encountered in high temperaturegalvanizing for example.

As it is generally known, a prerequisite for establishing a sufficienttempering resistance is the formation of a sufficient amount of Crcarbides, Mo carbides and/or V carbides, or carbon nitrides, whichprevent slip dislocation at elevated temperatures as a result ofprecipitation on the grain boundaries. This process is called secondaryhardening.

DE 44 46 709 A1 discloses the use of air-hardening steel for structuralhollow profiles of seamless, hot formed tubes, for example for doorreinforcements.

A steel alloy is hereby used having the primary elements: C(0.17-0.28%), Mn (1.30-2.50%), Si (0.30-0.49%), Cr (≦0.49%), Mo(0.20-0.40%), Ni (0.05-0.19%), Ti (0.02-0.07%), B (0.0015-0.0050%), Nb(0.01-0.10%), V(0.01-0.10%), and N (≦0.015%), remainder iron andincidental accompanying elements. In addition, the total content ofV+Nb+Ti may not exceed a value of 0.15%.

This alloy concept with additives of Nb and Ni is expensive for anair-hardening steel to meet the demanded requirements, and welding isproblematic because of the relatively high C content. Moreover, thissteel has a content of 0.30 to 0.49% of silicon which is critical forgalvanizing capability.

Then invention is based on the object to provide a high strengthair-hardening steel exhibiting excellent shaping properties, inparticular for the construction of lightweight vehicles, by using adifferent cost-saving alloy concept, and realizing the desiredmechanical properties while at the same time ensuring especially the HFIwelding capability as well as exhibiting superior general welding,galvanizing, and tempering resistances.

According to the teaching of the invention, this object is attained bysteel having the following contents in mass-%:

C 0.07 to ≦0.15 Al ≦0.05 Si 0.15 to ≦0.30 Mn 1.60 to ≦2.10 P ≦0.020 S≦0.010 N 0.0030 to ≦0.0150 Cr 0.50 to ≦1.0 Mo 0.30 to ≦0.60 Ti 0.010 to≦0.050 V 0.12 to ≦0.20 B 0.0015 to ≦0.0040,remainder iron including incidental steel-accompanying elements.

The high strength, air-hardening steel according to the invention forthe construction of lightweight vehicles is characterized in that thisalloy concept realizes an excellent welding capability during HFIwelding, without the undesired chromium-carbide precipitations in theweld seam. Compared to the known air-hardening steel for seamless tubes,there is also a reduced content of C and Mn to ensure a superior generalwelding capability while at the same time exhibiting excellent shapingproperties.

At the same time, the decreased Si content ensures the galvanizingcapability of the steel, and the addition of V ensures the temperingresistance.

Extensive laboratory tests have shown that the Cr content that iscrucial for the air-hardening effect can be lowered to the non-criticallevel in order to prevent the precipitation of chromium-carbide duringHFI welding, when the air-hardening capability of the steel issimultaneously improved again by a complex alloy concept on the basis ofCr—Mo—Ti—B.

The alloy concept in accordance with the invention is based on therecognition that, in contrast to conventional steel for seamless tubeswhere nitrogen must completely react with titanium to avoid boronnitride precipitations and to thereby ensure the effectiveness of theadded boron, nitrogen fixation may also be realized by other alloyelements such as Cr or Mo.

Establishment of an over-stoichiometric addition of titanium in relationto nitrogen is therefore no longer necessarily required. The addition ofvanadium triggers precipitations of vanadium-carbon-nitrides of typeV(C,N), when exposed to higher tempering temperatures, and opposes adrop in strength via a secondary hardening.

Tests have been conducted initially with two known air-hardening steelbased on different alloy concepts. Alloys on the basis of Cr—Mo—V(variant 1) as well as on the basis of Mn—Si—Ti—B (variants 2 and 3)were established for these comparative steels and their properties wereexamined.

Melt C % Si % Mn % N % Cr % Mo % Ti % V % B % Remark Variant 1 0.15 0.211.30 0.0056 1.40 0.58 — 0.19 — Cr—Mo—V Variant 2 0.10 0.71 2.16 0.00600.69 0.31 0.027 — 0.0034 Mn—Si—Ti—B Variant 3 0.14 0.53 1.98 0.0060 0.690.31 0.027 — 0.0034 Mn—Si—Ti—B Variant 4 0.10 0.48 1.60 0.0120 0.57 0.560.015 0.16 0.0034 V—Ti—B Variant 5 0.10 0.30 1.92 0.0120 1.15 0.63 0.0200.14 0.0032 V—Ti—B

Although variant 1 meets the demanded mechanic-technological properties,the high Cr content of 1.4% causes, as already described above, theundesired chromium-carbide precipitations and thus prevents theformation of a high-quality HFI weld seam and thus the production oflongitudinally welded tubes.

Variant 2 is used to date only for the seamless tube production inungalvanized configuration. This steel exhibits in particular a smallcontent of carbide-forming elements, primarily chromium.

Compared to variant 2, variant 3 has lower Si and Mn contents, whereasthe C content is slightly increased.

These alloy concepts based on Mn—Si—Ti—B have the drawback that theexcessive Si content, although necessary for realizing high strengthvalues, complicates component galvanizing. Furthermore, the materialstrength significantly deteriorates below the required levels attemperatures of about 550° C. so that the tempering resistance is alsonot ensured.

The variants 4 and 5, based on a V—Ti—B concept, exhibit in the “soft”state mechanical characteristic values which meet the requirements.

The tempering resistance of these variants can be improved in comparisonto the variants 2 and 3 by addition of V as well as by a slightlyelevated Mo fraction.

Variant 4 having, compared to the variants 2 and 3, a Mo content whichhas been increased to 0.56%, still fails to provide sufficient temperingresistance.

In order to improve the tempering resistance, the Mo content of variant5 has been slightly elevated to 0.63% and the Cr content significantlyto 1.15%. In the tempered state, the requirements have been met attempering temperatures of up to about 550° C., whereas above thistempering temperature a significant strength rise has been determinedwhich does not meet requirements.

On the basis of these recognitions, the alloy concept according to theinvention, as described above, has been established, with the followinganalysis range proven especially advantageous:

C 0.086 to ≦0.10 Al ≦0.05 Si 0.20 to ≦0.30 Mn 1.80 to ≦2.00 P ≦0.020 S≦0.010 N 0.0030 to ≦0.0125 Cr 0.70 to ≦0.80 Mo 0.40 to ≦0.50 Ti 0.020 to≦0.030 V 0.13 to ≦0.17 B 0.0025 to ≦0.0035,remainder iron including incidental steel-accompanying elements.

In a cold strip of 1.5 mm thickness, produced in accordance with thisalloy concept, as well as tubes having the dimensions 60×1.5 mm,properties have been determined in soft state as well as also intempered state, which meet the demanded mechanical requirements, on onehand, as well as requirements with respect to welding and galvanizingcapabilities.

The chemical composition of the steel was as follows:

C % Si % Mn % S % P N Cr Mo Ti V B 0.10 0.25 1.97 0.004 0.011 0.01250.72 0.50 0.028 0.15 0.0030

The determined mechanical characteristic values, as determined in thesoft state of the cold strip are listed in the following with respect todifferent tempering temperatures:

R_(el) or R_(p0.2) [MPa] R_(m) [MPa] A₈₀ [%] demanded State 330-500480-620 ≧20 soft 364 535 28.7 100° C./20 min 360 533 29.1 tempered 150°C.¹⁾ 374 532 26.7 250° C. 376 533 27.6 350° C. 434 538 25.7 450° C. 445538 25.3 550° C. 416 530 26 650° C. 422 523 27.9 ¹⁾Tempering time ineach case 15 min with subsequent air cooling

Tempering should hereby simulate the heat treatment from the state“soft” during hot galvanizing. The results show that the requirementswith respect to the steel properties in the soft state have been meteven after galvanizing.

The mechanical characteristic values, as determined in the temperedstate of the cold strip, are listed hereinafter in the event of heattreatment comprised of austenitizing for 15 min at 920° C., subsequentair cooling to room temperature, and final tempering to differenttempering temperatures:

R_(el) or R_(p0.2) [MPa] R_(m) [MPa] A₈₀ [%] demanded State 750-850850-1000 ≧11 tempered 350° C.¹⁾ 838 998 14.0 tempered 400° C. 817 99514.8 tempered 450° C. 835 985 15.8 tempered 500° C. 791 971 16.2tempered 550° C. 764 926 16.8 tempered 600° C. 784 940 17.1 tempered650° C. 850 962 18.1 tempered 700° C. 780 883 21.0 ¹⁾Austenitizing at920° C. 15 min + air cooling to room temperature + tempering 15 min withsubsequent air cooling.

The results show the high tempering resistance of the steel up totemperatures of at least 700° C.

The mechanical characteristic values as determined in the soft state ofthe tubes are listed hereinafter:

R_(el) or R_(p0.2) [MPa] R_(m) [MPa] A₈₀ [%] demanded State 330-500480-620 ≧22 soft 337-345 511-512 >37

The mechanical characteristic values as determined in the tempered stateof the tubes in the event of a heat treatment, comprised ofaustenitizing at 930° C., the subsequent air cooling, and tempering to575° C., are:

R_(el) or R_(p0.2) [MPa] R_(m) [MPa] A₈₀ [%] demanded State 700-850800-1000 ≧13 tempered 820-830 967-974 13.0

As further tests of the steel according to the invention have shown,this steel is not only advantageous for use in the automobile industrybut furthermore in all applications in which enameling in combinationwith high steel strengths is required. Advantageously, the enamelingtemperature of the steel of above 900° C. may hereby be used to obtainhigh steel strengths using air hardening during subsequent cooling. Thefield of application for such enameled, air-hardened steels may, e.g.,include the household appliance industry or the chemical apparatusconstruction industry.

The advantages of the air-hardening steel according to the invention arelisted again hereinafter;

-   very good cold-forming capability in soft state,-   very good welding capability in soft and air-hardened state,-   very good HFI welding capability,-   easy to coat by means of conventional coating processes, like    cathodic dip-coating (CDC), hot-galvanizing, and high temperature    galvanizing,-   very good enameling capability,-   use for welded, statically and dynamically highly stressed    structures,-   more cost-saving than comparable alloy concepts.

1. A high strength, air-hardening, temper-resistant steel suitable forwelding and galvanizing, comprising the elements (contents in mass-%): C0.07 to ≦0.15 Al ≦0.05 Si 0.15 to ≦0.30 Mn 1.60 to ≦2.10 P ≦0.020 S≦0.010 N 0.0030 to ≦0.0150 Cr 0.50 to ≦1.0 Mo 0.30 to ≦0.60 Ti 0.010 to≦0.050 V 0.12 to ≦0.20 B 0.0015 to ≦0.0040,

remainder iron including incidental steel-accompanying elements.
 2. Thesteel of claim 1, wherein the steel has a C content of 0.08 to ≦0.10%.3. The steel of claim 1, wherein the steel has a Si content of 0.20 to≦0.30%.
 4. The steel of claim 1, wherein the steel has a Mn content of1.80 to ≦2.0%.
 5. The steel of claim 1, wherein the steel has a Ncontent of 0.0030 to ≦0.0125%.
 6. The steel of claim 5, wherein thesteel has a N content of 0.0030 to ≦0.00805%.
 7. The steel of claim 1,wherein the steel has Cr content of 0.70 to ≦0.80%.
 8. The steel ofclaim 1, wherein the steel has a Mo content of 0.40 to ≦0.50%.
 9. Thesteel of claim 1, wherein the steel has a Ti content of 0.02 to ≦0.03%.10. The steel of claim 1, wherein the steel has a V content of 0.13 to≦0.17%.
 11. The steel of claim 1, wherein the steel has a B content of0.0025 to ≦0.0035%.